Electrically insulating bodies



June 22, 1943. l. w. A. KiRKwoOD 'F.-rAL 2,322,214

EL-ECTRICALLY INSULATING BODIES Filed Junel, 1939 2 Sheets-Sheet 1 y m)4, MVM

v Wto/weg v Juney22, 1943. L'w. A YKIRKwooD E-rAx. 2,322,214

ELECTRICALLY INSULATING' BODIES vFiled June i3, 1939 2 Sheets-Sheet 2vPatented June 22,

ELECTRICALLY INSULATING BODIES Ian Ward Anderson Kirkwood and PatrickDunbar Ritcln'e, Newcastle-on-Tyne, England, assignors to A. Beyrolle &Company Limited, Hebburn-on-Tyne, England, a company of Great BritainApplication June 13, 1939, Serial No. 278,965 In Great Britain June 14,1938 20 Claims.

This invention relates to electrically insulating bodies, such forexample as insulating bushings or sleeves.

Electrical insulators which consist of superimposedV layers of acellulosic base, such for example as paper br cloth, bonded withsynthetic resins or other binders are open to the difficulty thatI theyare liable to absorb moisture, since cellulosic materials, in general,are inherently hygroscopic. Even if the cellulosic insulator isimpregnated with a synthetic resin the absorption of moisture by thefibres will not-be wholly prevented and there will ultimately be adecrease in the dielectric strength, increase of power facl tor, andswelling of the insulator at the exposed ends of the fibrous layers.`

An object of the invention is to overcome this difficulty' by winding orwrapping fibres of a vitreous substance, for example glass-wool, pref,-erably felted or inthe form of a woven fabric, in a number ofsuperimposed layers about a conductor, or about a former which may incertain cases be a conductor, and filling the interstices with anelectrically insulating gas, or with an insulating medium which, duringthe filling process, isin a liquid, semi-liquid or dissolved state.

It is desirable that the non-crystalline substance employed be uniformin structure and free from incorporated -foreign matter, that it willprovide individual fibres of a satisfactory textile length, and that itwill be wholly (or substantially wholly) non-hygroscopic.

In order to facilitate filling of the interstices between the fibreswith the insulating medium the fibres may be bound together in opentexture by a preliminary binder prior tofilling of the interstices withthe insulating medium.

According to a further object of the invention the interstices in thefibrous layers are filled with an insulating binder, such for example asa phenol-formaldehyde resin, a higher chlorinated naphthalene,vorshellac applied in solution or, in certain cases where this is possible,in liquid or semi-liquid state. The insulating binder may be applied tothe fibres prior to the winding or wrapping which is then effected underthe application of pressure and/or heat so that the layers areimpregnated with the binder which binds the layers into a solid mass.Alternatively, the fibres (preferably bound in open mesh by apreliminary binder as above described) are first wound or wrapped andthe interstlces are then filled with the insulating binder, as bydipping the wound or wrapped layers in the insulating binder while thisis in a liquid or semi-liquid state.

According to yet a further object of the invention the interstices inthe wound or wrapped layers are filled by impregnation, preferably invacuo, with a polymerisable liquid, such for example as styrene, thisliquid being then poly-l merised by heating, if necessary underpressure. Thus, for example, the liquid impregnant may be forced throughthe insulating mass from end to end'so as to force out the air and fillthe interstices with the impregnant.

With 'a view to imparting flexibility as may be desired a suitableplasticiser may, in any of the above methods, be incorporated in theinsulating l binder.

Instead of binding the fibres into a solid mass, the Vwound or wrappedlayers may be arranged within a suitable casing containing an insulatingliquid such as hydrocarbon oil, or a liquid chlorinated hydrocarbonsuch, for example, as chlorinated diphenyl, this insulating liquidremaining in the liquid state and filling the interstices in the woundor wrapped layers.

chloro-paraflln, or nitrogen, an argon, the wholev being enclosed in agas-tight casing or jacket.

In the accompanying drawings,

Figure 1 illustrates one method of forming a condenser bushing accordingto the invention;

Figure 2 shows the finished bushing in longitudinal section;

Figure 3 illustrates another method according to the invention;

Figure 4 shows the finished bushing in longitudinal section. and

Figure 5 illustrates a modified form of finished bushing.

- In Figure lthe bushing is formed by applying to a cord or a strip offelted glass wool or woven glass cloth, for example a glass wool fabric,a

binder constituted by phenol formaldehyde resin,

polystyrene or polyvinyl acetal. The strip or cord A thus treatedorfvamlshed is dried and then wrapped in a number of superimposed layers`A1 about a former B, which may be a conductor, with the insertion ofstress-grading foils C bebetween the layers A1 as they are wound. The

layers A1 with the foils C between them are subjected to heat andpressure by rolls D so that the interstices in and between the layers A1are impregnated with the binder and the insulation is thus bound into asolid mass about the former or conductor B as shown in Figure 2.

In accordance with a second method, as shown in Figure 3, a cord or astrip of felted or woven glass wool A3 is iirst wrapped round aconductor or former B1 in a number of superimposed layers A4,stress-grading elements Cl being inserted between the layers A4 as theyare wound. The layers are then impregnated, preferably in vacuo, with apolymerisable liquid such for example as styrene or methylmetchacrylate, or solutions therein of the corresponding polymers. Theliquid is then polymerised by the application of heat, if necessaryunder pressure to avoid formation of bubbles. The nished bushing isshown in Figure 4.

According to a modified method the cord or strip of woven or feltedglass wool A (Figure 1) wound in a number of superimposed layers about aconductor or former with stress-grading foils C between them, is dippedor otherwise impregnated, preferably in vacuo, with a molten wax-likematerial, such for example as chlorinated naphthalene or ceresine, whichon cooling will set to bind the bres into a solid mass.

According to another modification a cord or strip of glass cloth, forexample felted or woven glass wool has applied thereto shellac, a.natural wrapped in a number of superimposed layers r about a conductoror former as described with reference to Figure l, metal foils beinginserted between adjacent layers as they are wound and subjected tosimultaneous heat and pressure by the rolls D.

In order to impart iiexibility as may be desired a suitable plasticiser,such for example as chierinated diphenyl, amyl naphthalene, dibutylphthalate, or tricresyl phosphate, may in each of the above methods beincorporated in the binder.

Instead of binding the fibres into a solid mass as previously described,the layers B2 `may, as shown in Figure 5, be wound or wrapped around theconductor A2 (with interposed stress grading foils C2) and then insertedin a casing D consisting of porcelain or other suitable material andprovided with a ange D1 for fixing it to apparatus casing D2. The casingcontains an insulating liquid such as hydrocarbon oil or a liquidchlorinated hydrocarbon such, for example, as chlorinated diphenyl, thisinsulating liquid remaining in the liquid state and filling theinterstices in the wound or Wrapped layers. Grooves D3 may be providedin the interior wall of the ilange D1 to assist in the circulation ofthe insulating liquid, the interior of the casing being connected by apipe D4 to a supply of the liquid so as to permit breathing and tocompensate for air leakage.

Alternatively the casing D of the bushing shown in Figure 5 may beiilled with an insulating gas at or above atmospheric pressure, the gasfilling the intcrstices of the wound or wrapped layers B2 and having adielectric strength within the interstices substantially greater thanthat of air at atmospheric pressure. D may be lled withdichloro-difiuoro-methane or other `gaseous iluorinated chloro-parai'neor nitrogen or argon.

impregnation of the interstices between the nbres will be facilitated ifthe texture of the For example, the casing r fibrous layers is as openas may be practicable. Though it is a matter of some difficulty to weavea fibre-glass textile of loose open mesh owing to warp-slip, thisdifiiculty may be overcome by using, for example in any of the methodsabove described, a felted mat of glass fibres held together by a smallquantity, say 1% to 2% by weight, of a preliminary binder such forexample as phenolformaldehyde resin or polystyrene, the binder contentbeing just suflicient to hold the libres against slip at their mainpoints of mutual contact. This gives an open-textured material whichwill be readily permeated by the liquid impregnant. Alternatively afelted mat as described may be employed, for example with the iirst andfourth methods above described, in which the preliminary binder contentis high, for example 40% to 60% by weight of the whole, so that, afterwinding or wrapping, impregnation of the felted mat by thebinder can beeiected by hot-pressing or hot-rolling the superimposed layers. If athermoplastic binder is employed with this method careful control of thetension to which the material is subjected during winding or wrappingshould be exercised in order to avoid undesirable stretching.

The felted mat, incorporating the binder, may be formed for example byagitating a suspension of short bres of a glass-like substance innon-crystalline form (e. g. glass-wool) in a reaction mixture givingrise to a synthetic resin. For example, the fibres may be stirred into aheated mixture of phenol and aqueous formaldedyde in suitableproportions. When the condensation has proceeded to the point where thereactants have formed an A-stage phenolic resin (resole) the whole ispoured out on to a smooth, endless conveyor belt, forming a thin filmfrom `which all surplus water is carefully evaporated,

if desired under reduced pressure. The resulting lm may then be woundinto a bushing in the uaial manner under the action of heat andpressure.

It will be understood that the electrical, me-

chanical, and other properties of the insulating body produced inaccordance with the invention may be modified to suit requirements andconditions oi use, by suitable selection not only of the vitreousnon-crystalline material but also the ller or binder. Thus, a lead glasswill usually give a lower power factor and a higher di electric constantthan a sodalime glass, but both possess a surface which is somewhatsusceptible to atmospheric moisture. The thickness of the layerofVmoisture which can be adsorbed on the surface of the fibres willdepend upon their chemical composition, such thickness usuallydecreasing, for example, with decrease of the soda and increase of theboric oxide content of the libres. It is in general preferred to employthe substantially alkali-free boro-silicate glass fibres alreadyavailable in the industry, but a particularly desirable combination ofmaterials, from the dielectric standpoint, comprises vitreous silicalibres impregnated or bound with a hydrocarbon such as polystyrene, or apolymerized ethylene plastic. When employing woven fibres the warp maydiffer chemically or otherwise from the weit, so as to produceintentional differences, in two mutually perpendicular directions, ofvarious characteristics, such for example as elasticity. When the lowestpossible power-factor is required, it is desirable that the iibres be asfree as possible from the emulsions oi oil, starch, and other matertalswith-which `they are usually moistened during spinning into thread form.These materials usually contain emulsifying agents which, though presentonly in minute amount on the final weave, are polar compounds whichcould with advantage be removed, prior tothe application of the binderor impregnant, for examplebyl` washing with water, trichlorethylene orother suitable solvents or vapours thereof, followed by intensivedrying.

The vitreous substance employed in accordance with the invention has noinherent tendency to absorb. moisture and, though it may tend to adsorba surface layer of moisture from a I. humid atmosphere, this layer canreadily be remoyed by heating prior to the fibres being filled or'boundto form the bushing. For this purpose. by using an inorganic glass-likenoncrystalline substance, a higher temperature can safely be employedthan with cellulosic materials which will only withstand a comparativelylow temperature. Further, whereas a cellulosic material, such as' paper,coated with polystyrene, can be pressed into laminated sheets attemperatures of about 120 C. to 160 C., paper coated with polymerlzedN-vinyl carbazole would require a temperature of about 250 C. to 300 C.in order to render the binder sufficiently plastic to flow readilylntothe interstices. 'I'his would, however,'damage .the paper and renderit brittle. By employing an inorganic vitreous substance instead ofcellulosic material much higher manufacturing temperatures can safely beapplied. Again, insulators composed of cellulosic materials bound withphenolic resins are readily tracked, that is to say they produce acarbonized conducting track when brought into contact with an electricarc. This tendency can be reduced by substituting a urea fora phenolicresin, or glass for cellulosic material. It can be substantiallyeliminated in an insulating body according to the invention using glassfibres bound with a urea or other non-'tracking plastic since an arcwill merely cause fusion of the surface fibres.

The vitreous material in the form of a tape or cord may be woundhelically about the former or conductor. For example, the invention maybe applied for the production of a long insulated conductor by helicallywinding about the conductor a wovenst'rip or a cord of glass-wool, to

- which a phenol-formaldehyde resin has been applied according to thefirst method described. 'Ihe wound helical layers are then subjected toheat and pressure so that the interstices therein are filled with thephenol-formaldehyde binder. Alternatively the tape or cord may first behelically wound-about the conductor and the layers impregnated withliquid styrene or methyl methacrylate which is then polymerised,according to the second method described. The tape or cord may also beemployed for the manufacture of insulated articles, such for example ascurrent transformers, having a .complicated shape.

.It will be understood that the application of the insulating filler orbinder may be effected either before or after winding or wrapping of thesuccessive layers of the vitreous substance. For example, the vitreoussubstance may be varnished -with a thermo-setting varnish before windingon the conductor or former, or the layers may be wound before theinterstices are filled, say by a monomer which is subsequentlypolymerized, or a syrupy solution of a polymerA in a monomer may beapplied to the glass-like substance before winding the layers on theconductor or former and then polymerising.

In any of the methods above described the filler or binder may beapplied as an emulsion or suspension 'of globules or particles cf thelling or binding medium in an immiscible liquid.

The superimposed layers may be formed from a textile fabric woven fromspun fibres or threads of the glass-like substance in non-crystallineforni. These layers may also be formed by winding thin fibres, sheets,irregular layers similar to felt, or extruded woven or braided cord,made of the said glass-like substance.

What we claim as our invention and desire to secure by Letters Patentis:

l. The method of forming an electric condenser bushing, 'comprisingwrapping fibres of a vitreous substance previously woven into the form.'sure.

2. The method of forming an electric condenser bushing about aconductor, comprising winding fibres of glass-wool previously woven intothe .form of a fabric in superimposed layers about the conductor,successively inserting separate stress-grading layers between adjacentfibrous layers as they are wound, and filling the inter' stices in thelayers with an insulating medium whose dielectric strength Within theinterstices exceeds that of air at atmospheric pressure.

3. In the method as claimed in claim 1, `the additional step of bindingthe fibres together in open texture by 'a preliminary binder prior tofilling of the interstices with the insulating medi- 4. The method offorming an electric condenser bushing, comprising wrapping fibres of avitreous substance in superimposed layers about a former, successivelyinserting separate stressgrading foils between adjacent layers duringthe winding, and filling the interstices in the layers with a solidinsulating binder which, during the filling process, is in a fluidstate.

5. The method of forming an electric condenser bushing, comprisingapplying an electrically insulating binder to fibres of a vitreoussubstance, winding the bres in superimposed layersabout a former,inserting at least one stressgrading foil between successive layersduring the winding, and subjecting the layers so formed to heat to causethe interstices in the layers to be a the filled layers to heat whichpolymerises the filler.

8. The method of forming an electric condenser bushing, comprisingweaving fibres of glass-wool into the form of a fabric winding thefabric in superimposed layers about a former, inserting separatestress-grading conducting layers between adjacent fabric layers duringthe winding, filling interstices in the layers with a polymerisableliquid ller, and subjecting the lled layers to heat which polymerisesthe ller.

9. The method of forming an electric condenser bushing about aconductor, comprising rst treating a strip of woven glass-wool fibreswith a binder including a plasticiser, wrapping the strip in a number ofsuperimposed layers about the conductor, inserting stress-grading foilsbetween adjacent layers during the winding, subjecting each layer as itis wound to heat and pressure whereby the binder is caused to ll theinterstices in the layers thus binding the whole into a solid mass aboutthe conductor.

10. The method of forming an electric condenser bushing about aconductor, comprising wrapping a strip of felted or Woven glass-woolfibres in a number of superimposed layers about the conductor, insertingat least one conducting layer between adjacent felted layers during thewinding, impregnating the layers with a polymerisable liquid ller,subjecting the whole to heat and pressure, polymerizing the liquidfiller and thus binding the layers into a solid mass about theconductor.

11. The method of forming an electric condenser bushing, comprisingapplying `an electrically insulating resinous binder to glass-woollibres, winding the fibres in superimposed layers about a former,inserting separate stress-grading foils between adjacent layers duringthe winding, and subjecting the layers so formed to heat which causesthe interstices in the layers to be illed by the binder.

12. The method as claimed in claim 11, in which the binder comprisesphenol-formaldehyde resin.

13. The method as claimed in claim l1, in which the binder comprises anunsaturated aromatic hydrocarbon of the vinyl type.

14. The method as claimed in claim 11, in which the binder comprisespolyvinyl acetal.

15. The method as claimed inelaim 9, in which the plasticiser compriseschlorinated diphenyl.

16. The method as claimed in claim 9, in which the plasticiser comprisesamyl naphthalene.

17. The method of forming an electric condenser bushing comprisingpreparing a suspension of short glass-Wool fibres in a reaction mixturegiving rise to a synthetic resin, agitating the same, pouring thesuspension on to a flat belt thereby forming a lm-like strip, Windingsaid strip in superimposed layers on a former and subjecting the Wholeto heat.

18. The method of forming an electric condenser bushing7 which comprisescreating a suspension of short glass-Wool fibres in a reaction mixturecomprising phenol and aqueous formaldehyde and giving rise to asynthetic resin, agitating until condensation has cuased the reactantsto form an A-stage phenolic resin, pouring the whole on to a flat beltthereby forming a nlm-like strip, winding said strip in superimposedlayers, inserting at least one stress-grading conducting layer betweenadjacent film layers during the winding, and subjecting the wound layersto heat which causes the interstices in the layers to be filled by thesynthetic resinous binder.

19. The method of forming an electric condenser bushing, which comprisesWinding bres of a vitreous substance in superimposed layers, insertingat least one stress-grading foil between successive layers during thewinding, filling the interstices in the layers with an insulating binderin the form of an emulsion and subsequently solidifying said emulsion.

20. The method of forming an electric condenser bushing, which compriseswrapping bres of a vitreous substance in superimposed layers, insertingat least one stress-grading foil between successive layers during thewinding, filling the interstices in the layers with an insulating binderin the form of a suspension and subsequently solidifying saidsuspension.

IAN WARD ANDERSON KRKl/VOOD. PATRICK DUNBAR RITCHE.

